What is a substitute for corn starch?

I almost always use arrowroot. It's almost always available in bulk at the grocery store (with the spices) and substitutes 1:1 for corn starch. Tapioca powder works too, but is more expensive and I find that it has more of a tendency to clump.

Tapioca flour and arrowroot could be subbed depending on what you're making; I often use tapioca flour in place of cornstarch when baking a fruit pie.

Kosher potato starch. You can also use flour to thicken or instant mashed potato flakes.

What are you making? What we suggest might differ, depending on the dish. ;o)